The invention pertains to a method for the production of precision castings by the centrifugal casting, with controlled solidification, of a melt under vacuum or shield gas into a preheated mold with a central gate and several mold cavities extending toward the outside periphery of the mold, the mold cavities being surrounded by a material or a material combination with a coefficient of thermal conductivity which is lower than that of copper.
There is an increasing demand for components of titanium or alloys containing large amounts of titanium, because these materials have a low specific weight and yet are extremely strong, provided that the specific properties of titanium are taken sufficiently into account, these properties including a high melting point and a considerable degree of reactivity at high temperatures. At its melting temperature, titanium reacts not only with reactive gases, including oxygen in particular, but also with oxides and nearly all ceramics, because these usually consist at least predominantly of oxide compounds. Because titanium has a greater affinity for oxygen, oxygen is removed from the oxides, with the result that titanium oxides are formed. Some materials which have proven to be superior for use in certain areas are listed by way of example below:
pure titanium,
Ti 6 Al 4 V,
Ti 6 Al 2 Sn 4 Zr 2 Mo,
Ti 5 Al 2.5 Sn,
Ti 15 V 3 Al 3 Cr 3 Sn
Ti Al 5 Fe 2.5
50 Ti 46 Al 2 Cr 2 Nb, and
titanium aluminides.
Worthy of particular mention is the use of titanium aluminides e.g., TiAl, as materials for numerous types of components. Because of their low density, relatively high high-temperature strength, and corrosion resistance, the titanium aluminides are considered an optimum material in various areas of application. Because these materials are very difficult to shape, the only practical method of forming them is to cast them. Especially in the case of casting, however, titanium-containing metals present another set of problems, which will be discussed in greater detail below.
Some examples of ways in which titanium-containing materials are used are listed below:
valves for internal combustion engines,
turbine rotors and turbine vanes,
compressor rotors,
biomedical prostheses (implants), and
compressor housings in aircraft construction.
Both intake and exhaust valves of certain titanium alloys have been found to be extremely reliable, especially in automobile racing, with the result that thought is being given to the mass production of such valves for internal combustion machines of all types.
EP-0 443 544 B1 deals with the problem of improving the dimensional accuracy or accuracy of shape of centrifugal casting molds of copper and the removability of workpieces of titanium alloys from the molds by adding zirconium, chromium, beryllium, cobalt, and sliver as alloying elements to the copper, the sum of all alloying elements together not exceeding 3 wt. %. A comparison example in which the copper was alloyed with 18 wt. % of nickel did not lead to success. The publication in question discusses the electrical conductivity of the material but not its thermal conductivity, so that the problems involving a high quenching rate and the formation of shrinkholes and pores are not treated. On the other hand, this literature reference does discuss the disadvantages of mold materials consisting of ceramic or oxide materials.
DE 44 20 138 A1 also describes a method of the general type described above. From this document and DE 195 05 689 A1, molds for implementing such methods are known, in which at least the surfaces of the mold cavities which come in contact with the melt consist of a material selected from the group consisting of tantalum, niobium, zirconium, and/or an alloy A with at least one of these metals, i.e., materials with a thermal conductivity which is considerably less than that of copper and also with a specific heat capacity which is much less than that of copper. Insofar as base materials for these mold cavity surfaces are discussed, the base bodies consist of different metals in the case of the object of DE 44 20 138, but the condition remains fulfilled that the thermal conductivity and the heat capacity of the complete mold are lower than the corresponding values of copper. DE 195 05 689 A1 recommends materials from the group consisting of titanium, titanium alloys, titanium aluminide, graphite, and silicon nitride as base materials for the molds. These base materials have the advantage of a much lower specific weight and are therefore especially suitable for centrifugal casting molds.
With the methods and apparatuses according to DE 44 20 198 A1 an DE 195-05,689 A1, it has already become possible successfully to produce precision castings from quenching-sensitive materials on a large industrial scale. In these methods, the goal is significantly to reduce the high quenching rate, desired in the past as a way of avoiding reactions with the mold materials, and thus to reduce significantly the formation of shrinkholes, voids, pores, etc. in the castings, and especially to avoid the need for expensive reprocessing by high-pressure compaction (HIP method) and/or welding. To reduce the quenching rate even more, the two last-cited publications recommend that the molds be preheated to a minimum temperature of, for example, 800xc2x0 C. For this purpose, it is provided that the mold is heated from the outside periphery; that is, the mold described in DE 44 20 138 A1 is surrounded by a heating cylinder. Because the walls of the gate must also reach the required temperature, it is necessary to heat up the entire volume of the mold to the temperature in question; and then, because the mold must also be cooled, it is necessary to cool its outside peripheryl by means of a gas with good thermal conductivity.
The known solutions are therefore energy-intensive and time-consuming, and the migration of the solidification front within the castings remains in a certain sense left to chance and/or depends to a considerable extent on the volume distribution of the castings. It is desirable for the solidification to occur in a controlled manner in the direction of the gate, so that the melt still present in that area can fill up any voids which may be forming in the casting.
The phrase xe2x80x9ccontrolled solidificationxe2x80x9d is more comprehensive than the phrase xe2x80x9coriented solidificationxe2x80x9d, because the goal is not so much to create a certain preferential direction of the individual crystals but rather to control the direction in which the solid/liquid solidification front migrates.
The book by Kurz and Samm entitled Gerichtet erstarrte eutektische Werk stoffe [Eutectic Materials with Oriented Solidification], Springer-Verlag, Berlin-Heidelberg-New York, 1975, pp. 195-198, describes how relative motion can be brought about between a beating device and an individual casting mold located coaxially inside it. No heating rate is given, and the rate at which the casting mold is moved is the same as the rate at which the solidification front of the melt migrates.
The invention is therefore based on the task of providing a method of the general type described above Which makes it possible to reduce the amount of energy required and to achieve shorter cycle times and which also promotes solidification from the outside toward the inside, that is, in the direction of the gate.
According to the invention, the task described above is accomplished in conjunction with the method described above in that, before the melt is poured, the mold is heated, starting from the gate, until the gate reaches a temperature which is a function of the material being cast, the heating being carried out at a rate sufficient to produce a temperature gradient of at least 100xc2x0 C. between the inside periphery and the outside periphery of the mold, the temperatures falling from the inside toward the outside.
The fundamental idea of the invention is based on a synergistic effect of the mold material and the heating direction. The use of a mold known in and of itself made of a material or a material combination with a coefficient of thermal conductivity lower than that of copper makes it possible, by heating the mold from only one side, to develop a very steep temperature gradient, the steepness of the gradient obviously also depending on the amount of heating power applied, the mass to be heated, and the heat losses in the direction of the unheated surfaces.
Heating the mold by starting from the gate and proceeding outward, which is the reverse of the state of the art, has the effect that the highest mold temperature is reached in the area of the walls of the gate, which means that the temperature, gradient decreases from the inside toward the outside. This has the quite considerable advantage that, during centrifugal casting, the walls of the mold which the overheated melt contacts at the end of its journey are colder than those which it contacts just before all of the melt has been poured. The solidification front therefore migratesxe2x80x94in a controlled mannerxe2x80x94from the outer end of the mold cavities or from the outside periphery of the mold toward the gate. As a result, melt still present in the gate can flow into the cavities to prevent the formation of shrinkholes, pores, etc.
The optimum temperature to which the walls of the gate are heated depends on or is determined by the material, but it can also be found by experiment. The most important point is that this temperature must have a falling gradient in the direction of the outside periphery of the mold, so that the effect described above is achieved.
It is especially advantageous for the temperature gradient to be adjusted to a value of 200-600xc2x0 C., preferably to a value of 300-500xc2x0 C.
When the method is used to produce precision castings of metal selected from the group titanium, titanium alloys with at least 40 wt. % of titanium, and superalloys, it is especially advantageous for the temperature of the walls of the gate to be adjusted to values of 600-1,000xc2x0 C. and for the temperature of the outside periphery of the mold to be adjusted to values of 300-600xc2x0 C.
It is also advantageous, when precision castings with different cross sections are being made, for the ends with the larger cross sections to be arranged pointing toward the gate.
Arranging the cavities this way in space is disadvantageous with respect to the most efficient utilization of the volume of a centrifugal casting mold, but the inward-pointing position of the ends with the larger cross sections reinforces the desired course of the solidification process, because these ends also have correspondingly larger volumes, and thus more liquid melt is available there for a longer period of time than in the narrower areas of the castings.
The invention also pertains to an apparatus for implementing the method described above, this apparatus being provided with a melting and casting device and with a chamber, in which a rotating mold with a central gate and several mold cavities extending from the gate toward the outer periphery of the mold and a heating device for preheating the mold are installed, the mold being made of a material or a material combination with a coefficient of thermal conductivity lower than that of copper.
To accomplish the same task, an apparatus according to the invention is characterized in that it has a device for producing relative motion between the heating device and the gate.
The heating device can advantageously be designed as a resistance heating body. It can be, for example, a hollow cylinder of graphite, which is slotted in such a way as to create a meander and which can be heated by the passage of current directly through it. A resistance heating body of this kind can be made appropriately narrow, so that it can be introduced into the gate. It is also possible, however, to design the heating device as an induction coil.
Molds such as those described in DE 4,420,138A1 and DE 195-05,689A1 can be used. As part of a further elaboration of the invention, however, it is especially advantageous for the mold to consist of stacks of forms arranged in several planes, the forms being provided with shoulders, by means of which they can be held on sector-shaped supports, after the forms and the supports have been arranged each in their own plane between spacer rings and after the stack of forms, supports, and spacer rings has been clamped by means of tension rods to a support plate, which is connected in a torsion-proof manner to the rotational drive unit.
A mold of this type is thus designed in modular fashion; that is, the forms can be replaced bad others with different mold cavities without the need to keep complete disks with their machined-in mold cavities in stock, as is the case in accordance with the state of the art.
It is also advantageous for the stack of forms, supports, and spacer rings to be surrounded by a clamping body, especially when the clamping body is made up of individual clamping rings, which overlap each other partially in the axial direction.
Here the object of the invention offers yet another special advantage, both with respect to the management of the method and also with respect to the apparatus or mold.
In the case of a centrifugal casting mold, the maximum radial and tangential tensile stresses occur at the outer periphery of the mold. They are a function of the diameter and rotational speed of the mold. On the one hand, it is desirable to use the highest possible rpm""s in order to produce a dense structure; for example, in the case of a mold with an outside diameter of approximately 500 mm, a speed in the range of approximately 800 rpm would be used. Calculations based on the mold materials in question, however, have shown that molds with high outside temperatures according to the state of the art in the dimensions cited can at best be operated at a maximum of 500 rpm. The creation, according to the invention, of a temperature gradient which decreases from the inside toward the outside, however, leads to the additional advantage that, because of the much greater strength of the mold materials at these temperatures, it is possible to work at much higher rotational speeds. For example, for a mold with the indicated dimensions, it is possible to work at 800 rpm or more, as a result of which the structure of the precision casting can be significantly improved. Simultaneously, the danger of the deformation of the mold at the outer periphery is significantly reduced.
Thus, for example, materials such as 800 H (iron-based alloy with 21% chromium and 32% nickel) or 80 A (nickel-based alloy with 19.5% chromium, 2.5% titanium, and 1.3% aluminum) can be used for the clamping body or clamping rings described above to clamp the supports and spacer rings. These are relatively inexpensive construction materials for machinery. The actual forms or form halves can consist of niobium, tantalum, zirconium, and/or alloys the reof, but they can also consist of alloys of these metals with additional metals or of base bodies with appropriate surface coatings or of shell-shaped liners of these materials.